1. Field of the Invention
The present invention generally relates to organic compounds in which a non-natural heteroatom is part of a 5-member ring, and particularly of nucleo-bases and their analogs comprising such 5-membered rings. More particularly, the invention relates to such compounds wherein phosphorus is a heteroatom.
2. Description of the Related Art
Phosphorus is not found free in nature, it always occurring in the fully oxidized state as phosphate. It is widely distributed in the phosphate form in all living cells, and plays a vital role in life processes such as energy storage and utilization in the form of nucleoside triphosphates. There is no known organism in which the chemistry of phosphorus is not utilized. The highly polymerized phosphate esters (such as are found in nucleic acid polymers, DNA or RNA) are the normal constituents of all cells. Although phosphorus is present in all nucleic acids as the oxidized phosphate form, the presence of phosphorus as a hetero atom replacing nitrogen in the purine base of nucleic acids, for example, is not yet known.
Several phosphono pyrimidine/purine nucleosides in which the phosphono group is attached directly to a carbon atom of the heterocycle, but not incorporated into the heterocyclic ring, have been reported (1, 2). Diethyl 6-chloro-9-(.beta.-D-ribofuranosyl)purine-8-phosphate, having the following structural formula (compound 1): ##STR2## exhibits significant activity against vaccinia, vesicular stomatitis and coxsackie B4 viruses in culture. This compound is also inhibitory to the proliferation of the murine leukemia (L1210), murine mammary carcinoma (FM3), human B-lymphoblast (Raji) and human T-lymphoblast (Molt-4F) tumor cell lines in culture (1).
The synthesis of 1,3-benzazaphosphole, having the following structural formula (compound 2): ##STR3## is described by Issleib et al. (3). 1,3-Azaphospholo[1,2-a]pyridine and 1,2,4-diazaphospholo[1,5-a]pyridine have also been synthesized (4). These studies have shown that such condensed systems containing trivalent phosphorus are much more stable than had been previously known. They are not attacked readily by dilute mineral acids and alkalies (3). Further, 3-methyl-1,3-benzazaphosphole, prepared by alkylation of the lithium salt of 1,3-benzazaphosphole with methyl iodide, and having the following structural formula (compound 3) is stable. ##STR4##
A single crystal X-ray diffraction analysis of 1,5-dimethyl-1,2,4,3-triazaphospholine, which has the following structural formula (compound 4): ##STR5## shows that both of the P-N distances in that molecule are identical, suggesting a P.pi.N.pi. electron delocalized system in 1,5-dimethyl-1,2,4,3-triazaphosphole (5).
The synthesis of 1-.beta.-D-ribofuranosyl-1,2,4-lambda-3-diazaphosphole-3-thiocarboxamide, and its effect against L1210 in mice is described by Riley et al. (115).
Conventional nucleo-base analogs have previously been evaluated for their toxicity to normal cells as well as for specific therapeutic efficacy against certain tumors or viruses, for example, using assay systems based on the inhibition of cytokines such as TNF.alpha. or IL-1.beta., as well as HCMV inhibition, among other methods. For the purposes of this disclosure a "nucleo-base" means adenine, guanine, thymine, cytosine or uracil.
Tumor necrosis factor alpha (TNF.alpha.)
Tumor necrosis factor alpha (TNF.alpha.), a mononuclear phagocytic cell derived protein, was originally described as a product of activated macrophages and shown to display tumoricidal activity (6-9). Extensive research during the last few years has made it apparent that TNF.alpha. is a highly pleiotropic cytokine (10) and may play a role in tumorigenesis, septic shock, multiple sclerosis, cachexia, inflammation, autoimmunity, and other immunological and pathological reactions (11). There are two forms of TNF.alpha., a type II membrane protein of relative molecular mass 26,000 (26 kDa) and a soluble, 17 kDa form generated from the cell-bound protein by proteolytic cleavage. Several different types of tumors have been described in which TNF.alpha. acts as an autocrine growth factor, including leukemias, ovarian tumors, renal cell carcinoma, breast adenocarcinoma, neuroblastoma, and glioblastoma (12-16). These tumors express TNF.alpha. and its receptors and furthermore proliferate in response to the cytokine. Agents that can block the production of TNF.alpha. in these different tumor cell types may have potential as antitumor agents.
Cytokines such as TNF.alpha. and interleukin-1.beta. (IL-1.beta.) also play a central role in the regulation of the immune system and they have been implicated in inflammatory processes as well as in the pathogenesis of many diseases (17, 18). TNF.alpha., first associated with tumor regression and with cachexia accompanying chronic invasive diseases, is now established as an immune modulator in normal and chronic inflammation situations as well as with septic shock (19). Deregulated production of TNF.alpha. in humans is thought to contribute to the development of diseases such as cancer-associated cachexia (20), endotoxic shock (21), graft versus host disease (22), autoimmunity (23) and rheumatoid arthritis (24, 25). Therefore, agents that can inhibit the production or maturation of TNF.alpha. and IL-1.beta. in these different indications may have excellent therapeutic potential.
Since the role of TNF.alpha. and IL-.beta. in the development of the septic shock syndrome and other ailments has been recognized (26), attempts have been made to suppress the production of these pathogenetic factors. Phosphodiesterase (PDE) inhibitors (27) are potential agents for blocking the cytokine pathway due to their ability to suppress TNF production via elevation of intracellular adenosine 3',5'-cyclic phosphate (cAMP) (28). The suppression of TNF.alpha. production by adenosine and certain xanthine derivatives (e.g. pentoxifylline) (29) by inhibition of PDE activity has recently been demonstrated (30).
Human cytomegalovirus (HCMV)
Cytomegalovirus (CMV) is the largest and perhaps the most complex member of the herpesviridae family (31, 32). Infections with human cytomegalovirus (HCMV) are common and usually asymptomatic; however, the incidence and spectrum of disease in newborns and immunocompromised hosts establish this virus as an important human pathogen (33). HCMV infection has been detected in 0.5 to 2.5% of newborn infants and is the most common identified cause of congenital infection (34). Symptomatic infants (5-10% of infected infants) can die of complications within the first months of life but more commonly experience permanent neurological damage (35). Infections due to HCMV also represent a significant complication in bone marrow or organ transplant recipients (36, 37). For example after allogeneic bone marrow transplant, HCMV infections occur in approximately 60 to 70% of bone marrow recipients who were HCMV seropositive before transplantation or who were seronegative but received bone marrow from a seropositive donor. In either instance, the principle post-operative cause of death in this patient population is interstitial pneumonia developed from the active HCMV infection (38). Despite the development of new treatment regimens using antiviral drugs or human hyperimmune therapy, GMV pneumonia still has a fatality rate of approximately 50% (39-41).
One of the most widely used drugs today for HCMV infections is ganciclovir (DHPG) (42-44). Ganciclovir is a potent inhibitor of most human and animal herpes viruses in culture whereas much higher concentrations are needed to inhibit cell growth of normal cells. However, DHPG is toxic to bone marrow progenitor cells in culture which was predictive of DHPG's adverse effects in vivo in that most clinical toxicity is myelosuppression (45). This problem is most apparent in patients in need of long-term therapy such as those with AIDS, CMV retinitis and transplant recipients. Another concern for patients on long-term DHPG therapy is the development of DHPG-resistant strains of HCMV (46, 47). The second anti-HCMV drug approved for use against HCMV retinitis in AIDS patients is foscarnet (PFA). This compound is a broad spectrum antiviral agent with observed activity against all known human herpesviruses (48). However, treatment with foscarnet has resulted in nephrotoxicity, hypocalcemia, and seizures in some patients (49). Therefore, additional efficacious, non-toxic drugs for use against HCMV infections are needed.